This invention relates to directional filters having four ports and, more particularly, to a filter system employing a pair of filters connected in phase quadrature between two waveguides employing symmetric and antisymmetric radiators to develop directional characteristics using linear electromagnetic propagation modes within the pair of filters.
Filters are commonly employed in the processing of electromagnetic signals. For example, such signals may be obtained from an array of antenna elements positioned for receiving a microwave signal. A common construction of such filters employs a series of cavities constructed of a cylindrical wall closed off by end walls and having portions, with divider walls set within the cylindrical wall to define a series of cavities arranged, one behind the other. The walls are made of an electrically conducting material, typically a metal such as aluminum, brass or silver plated steel. The divider walls have apertures for coupling electromagnetic power between adjacent cavities. The dimensions of the cavities and the configuration of the coupling apertures, or irises, are selected to provide for a desired bandpass characteristic to signals propagating through the filter.
A situation of particular interest involves the filtering of several signals simultaneously to provide a set of filtered signals, this being followed by a combination of the filtered signals to provide a sum of the filtered signals. Such combination has been accomplished by the use of a waveguide manifold having several ports which connect with output ports of respective ones of the filters. In order to operate the manifold with the set of filters, it is important to ensure that signals outputted by individual ones of the filters do not interfere with the operation of other ones of the filters resulting in signal distortion. Therefore, the various filters must be electrically isolated from each other to insure proper combination of the filtered signals at the manifold.
One form of isolation which has been employed involves the use of four-port filters having a directional characteristic in the sense that a signal applied to one of two input ports exits from only one designated output port of a pair of output ports. With such an arrangement, the characteristics of the filter, which may be described mathematically by a scattering matrix, allow for the connection of several of the filters to one manifold without interference being introduced into the operation of one filter by the presence of another filter.
Heretofore, such a four-port directional filter has been built by use of circularly polarized filters in which the two components of right-hand and left-hand polarizations have allowed the implementation of a filter with the necessary scattering matrix to allow for a parallel combination of signals from several filters at a manifold. However, such circularly polarized filters have permitted design only in accordance with a Chebyshev characteristic (filter transfer function), but does not admit the well known design by quasi-elliptic linear mode transfer function.
Thus, a problem exists in that existing microwave filter structure which provide the desirable four-port directional characteristic are limited to circular polarization, with only a Chebyshev transfer function and do not admit implementation with filter structures operative with linear mode.